1. Field of the Invention
The present invention relates to a method of determining a recording error simultaneously with recording of data on a recording medium, and a data recording apparatus for executing this method.
2. Related Background Art
FIG. 1 shows an example of an optical head of a conventional magnetooptical recording/reproduction apparatus.
In FIG. 1, a semiconductor laser 22 serves as a light source. Divergent light emitted from the laser 22 is collimated via a collimator lens 23, and the light emerging from the lens 23 is guided to an objective lens 26 via a beam-shaping prism 24 and a polarizing beam splitter 25. The light is focused by the objective lens 26 to form a light spot on a magnetic layer of a magnetooptical recording medium 27. On the other hand, a magnetic head 28 applies an external magnetic field to the position of the light spot.
Light reflected by the magnetooptical recording medium 27 returns to the polarizing beam splitter 25 via the objective lens 26 again, and some light components of the reflected light are split by the splitter 25. The split light components are guided to a control optical system. In the control optical system, the split light components are further split into two beams by another polarizing beam splitter 29. One of the two split beams is guided to a reproduction optical system 30 to obtain a reproduction signal. The other beam is guided to a photodetector 38 via a focusing lens 36 and a half prism 37, and is guided further to a photodetector 40 via a knife edge 39, thereby obtaining auto-tracking and auto-focusing control signals for an optical head.
The reproduction optical system 30 comprises a half-wave plate 31 for rotating the direction of polarization of a beam through 45.degree., a focusing lens 32 for focusing a beam, a polarizing beam splitter 33 for splitting a beam into two beams, and photodetectors 34 and 35 for detecting the two beams split by the polarizing beam splitter 33. The reproduction signal can be obtained using differential detection signals output from these photodetectors 34 and 35.
FIG. 2 is a view for explaining how to obtain a magnetooptical signal.
A magnetooptical recording medium records information by utilizing a difference in direction of magnetization. When linearly polarized light is radiated onto the medium, the direction of polarization of the linearly polarized light rotates clockwise or counterclockwise depending on the difference in direction of magnetization.
For example, the direction of polarization of linearly polarized light incident on the magnetooptical recording medium is represented by a direction of a coordinate axis P shown in FIG. 2, reflected light for downward magnetization is represented by R.sub.+ rotated through an angle +.theta..sub.K, and reflected light for upward magnetization is represented by R.sub.- rotated through an angle -.theta..sub.K. When an analyzer is arranged in a direction, as shown in FIG. 2, light A corresponds to the reflected light R.sub.+ transmitted through the analyzer, and light B corresponds to the reflected light R.sub.- transmitted through the analyzer. When these light components are detected by a photodetector, information can be obtained as a difference in light intensity. In the prior art shown in FIG. 1, the polarizing beam splitter 33 serves as an analyzer. More specifically, the splitter 33 serves as an analyzer in a direction inclined from the axis P at an angle +45.degree. for one of two split beams, and also serves as an analyzer in a direction inclined from the axis P at an angle -45.degree. for the other beam. That is, since signal components obtained by the photodetectors 34 and 35 have opposite phases, a noise-reduced reproduction signal can be obtained by differentially detecting these signal components.
A magnetooptical recording/reproduction apparatus commercially available at present scans on an identical track three times upon recording of information. That is, the apparatus performs scanning for erasing previously recorded information, scanning for recording new information, and scanning for verifying whether or not new information is correctly recorded. The three scanning operations decrease the data transfer speed, and a demand has arisen for shortening the recording time.
As techniques for increasing the data transfer speed upon recording, the overwrite technique and direct verifying technique are known.
As the overwrite technique, a magnetic field modulation method and an optical modulation method are known.
In the magnetic field modulation method, as shown in FIG. 3, a magnetic head 41 and an objective lens 42 of an optical head are arranged to oppose each other to sandwich a magnetooptical recording medium 43 therebetween, and a magnetic field modulated according to a recording signal is applied by the magnetic head 41 in a state wherein a light spot having a predetermined intensity is radiated on the medium. The magnetooptical recording medium 43 used in the magnetic field modulation method has a structure wherein a protection & interference layer 47, a reproduction layer 46, a recording layer 45, and a protection layer 44 are stacked on a transparent substrate 48. FIG. 4 shows the characteristics of the coercive force with respect to the temperature of the recording layer 45 and the reproduction layer 46. In FIG. 4, the characteristics of the recording layer 45 are represented by a curve 49, and those of the reproduction layer 46 are represented by a curve 50. As can be seen from FIG. 4, the recording layer 45 has a large coercive force H.sub.C49 at room temperature and a low Curie temperature T.sub.C49, and the reproduction layer 46 has a smaller coercive force H.sub.C50 at room temperature and a higher Curie temperature T.sub.C50 than those of the recording layer 45. In FIG. 4, T.sub.comp represents the compensation temperature of the recording layer 45, H.sub.W represents the magnitude of the modulated magnetic field, T.sub.R represents the reproduction temperature, and T.sub.W ' represents the lower limit of the overwrite temperature.
The recording operation based on the magnetic field modulation method will be described below with reference to FIGS. 5A to 5D. Assume that the magnetooptical recording medium 43 shown in FIG. 3 is moved in a direction of an arrow B. In the magnetic field modulation method, a light spot shown in FIG. 5A is radiated at a light intensity having a predetermined power P.sub.W, as shown in FIG. 5C, thereby setting the temperature of a magnetic layer of the magnetooptical recording medium 43 to fall within a range between T.sub.W ' and T.sub.C50. In this temperature rise state, a magnetic field (FIG. 5B) modulated to .+-.H.sub.W according to a recording signal is applied by the magnetic head 41 to the radiation portion of the light spot. Thus, the direction of magnetization of the recording layer 45 is aligned in the same direction as the direction of the modulated magnetic field, and a domain is formed, thus allowing an overwrite operation. In this case, since the coercive force of the reproduction layer 46 is smaller than that of the recording layer 45, a domain corresponding to that of the recording layer 45 is formed in the reproduction layer 46. The domain has an arrow-headed shape, as shown in FIG. 5D, and its length W.sub.m is determined by a length W.sub.m ' of the modulated magnetic field.
On the other hand, in the optical modulation method, as shown in FIG. 6, an objective lens 53 of an optical head and a bias magnet 52 are arranged to oppose each other to sandwich a magnetooptical recording medium 54 therebetween, and a light spot whose intensity is modulated according to a recording signal is radiated from the optical head in a state wherein a magnetic field having a predetermined strength is applied from the bias magnet 52. The magnetooptical recording medium 54 used in the optical modulation method has a structure wherein a protection & interference layer 58, a recording layer 57, a recording auxiliary layer 56, and a protection layer 55 are stacked on a transparent substrate 59. An initializing magnet 51 is a magnet for initializing the recording auxiliary layer 56 of the magnetooptical recording medium 54.
FIG. 7 shows the characteristics of the coercive force with respect to the temperature of the recording auxiliary layer 56 and the recording layer 57. In FIG. 7, the characteristics of the recording auxiliary layer 56 are represented by a curve 60, and those of the recording layer 57 are represented by a curve 61. As can be seen from FIG. 7, the recording auxiliary layer 56 has a small coercive force H.sub.C60 at room temperature, and a high Curie temperature T.sub.C60. The recording layer 57 has a larger coercive force H.sub.C61 at room temperature and a lower Curie temperature T.sub.C61 than those of the recording auxiliary layer 56. In FIG. 7, T.sub.comp ' represents the compensation temperature of the recording auxiliary layer 56, H.sub.ini represents the magnitude of the initializing magnetic field, H.sub.W ' represents the magnitude of the bias magnetic field, T.sub.R ' represents the reproduction temperature, and T.sub.WL and T.sub.WH respectively represent the low- and high-level temperatures upon optical modulation in an overwrite operation.
The recording operation based on the optical modulation method will be described below with reference to FIGS. 8A to 8D. Assume that the magnetooptical recording medium 54 shown in FIG. 6 is moved in a direction of an arrow C. In the optical modulation method, the recording auxiliary layer 56 is magnetized in one direction at room temperature by the magnetic field H.sub.ini of the initializing magnet 51. Then, when an information recording portion of the magnetooptical recording medium 54 passes the bias magnet 52, the bias magnetic field H.sub.W ' is applied, as shown in FIG. 8B, and at the same time, a light spot shown in FIG. 8A is radiated from the optical head. The light intensity of the light spot is modulated to a low-level power P.sub.L and a high-level power P.sub.H according to a recording signal, as shown in FIG. 8C, and the temperatures at the information recording position are set to be T.sub.WL and T.sub.WH, accordingly. In this case, when the light power is the high-level power P.sub.H, since the temperature T.sub.WH of the recording auxiliary layer 56 exceeds the Curie temperature T.sub.C60, the direction of magnetization of the recording auxiliary layer 56 is reversed to the direction of the bias magnetic field. When the light power is the low-level power P.sub.L, the original direction of magnetization of the recording auxiliary layer 56 is maintained. In this manner, a domain is formed in the recording auxiliary layer 56, as shown in FIG. 8D, and is transferred to the recording layer 57 later upon cooling of a magnetic layer. The domain has a circular or elliptical shape.
With these overwrite techniques, erasing of previously recorded information and recording of new information can be attained by a single scanning operation.
The direct verifying technique is achieved in such a manner that two light spots are radiated onto a magnetooptical recording medium, the above-mentioned overwrite operation is performed using a leading light spot, and newly recorded information is immediately reproduced using a trailing light spot.
The conventional verifying method will be described in more detail below.
FIG. 9 is a schematic diagram showing an arrangement of a conventional magnetooptical disk apparatus. In FIG. 9, a magnetooptical disk 271 serves as a recording medium. The disk 271 is rotated by a motor (not shown), and is moved in a direction of an arrow D. An optical head and a magnetic head 282 are arranged at almost opposite positions to sandwich the disk 271 therebetween. The optical head includes a light source 272 such as a semiconductor laser, a collimator lens 273, a beam splitter 274, an objective lens 275, a focusing lens 276, an analyzer 286, a photodetector 277, and the like.
In the apparatus with the above-mentioned arrangement, a light beam emitted from the light source 272 is transmitted through the collimator lens 273 and the beam splitter 274, and is focused on a recording layer (magnetic film) of the magnetooptical disk 271 by the objective lens 275. In the light beam radiated portion of the magnetic film, the temperature is increased to a value near the Curie temperature of the magnetic film, and the coercive force is decreased. The magnetic head 282 applies a magnetic field in a direction perpendicular to the film surface to a portion near the light beam radiated portion of the magnetic film. The direction of magnetization of the portion with the decreased coercive force upon radiation of the light beam is aligned in the same direction as that of the applied magnetic field. The magnetic field to be applied from the magnetic head 282 is controlled by a magnetic head driver 281 to reverse its direction according to data to be recorded. Therefore, data is recorded on the portion scanned with the light beam upon movement of the disk 271 as a series of domains having upward magnetization and downward magnetization.
The data recorded as described above is read out after recording, and is subjected to error detection. Whether or not recording is performed normally is determined according to the number of detected errors. Such an operation is normally called "verification". When a recording error is determined by verification, data is re-recorded on a recording error portion or a portion different from the recording error portion.
In the verification, the light source 272 of the optical head emits a light beam having a lower power than that in recording, and the data recording portion on the disk 271 is scanned by this light beam. The direction of polarization of the light beam reflected by the disk 271 is modulated according to recorded data on the basis of a magnetooptical effect such as the Kerr effect. This reflected light is split by the beam splitter 274 from the optical path of the radiation beam, and is focused by the focusing lens 276. The reflected light transmitted through the focusing lens 276 is converted into light intensity-modulated according to the recorded data via the analyzer 286, and the converted light is detected by the photodetector 277. The output signal from the photodetector 277 is binarized by a binarizing circuit 278, and the binary data is input to a magnetooptical disk drive controller (to be abbreviated to as an ODC hereinafter) 280 via a data separator 279. The ODC 280 is connected to a microprocessor unit (MPU) 283 and a small computer system interface (SCSI) control circuit 284. In the ODC 280, errors of data read out from the disk are checked. When the number of errors is equal to or smaller than a predetermined determination criterion, the recording operation is ended. However, when the number of errors exceeds the determination criterion, a recording error is determined, and the ODC 280 instructs the device to perform re-recording of data, and the like.
FIG. 10 is a block diagram showing an arrangement of the ODC. The ODC performs data flow control, modulation, demodulation, synchronous processing, and the like, and an error correction code (ECC) circuit generates a parity in recording so as to correct an error generated in data and corrects, in reproduction, the error in data read out from the disk using the parity generated in recording.
In FIG. 10, the ODC is connected to the above-mentioned SCSI control circuit 284 by a DMA interface (I/F) 224 via a DMA controller 225. The ODC is also connected to the above-mentioned MPU 283 by an MPU-I/F 221 via an MPU-I/F circuit 222. The ODC also includes an ECC circuit 226, a DRAM controller 283, a buffer RAM 284, and a formatter circuit 227. These units are connected to each other via an internal bus 223. The formatter circuit 227 includes a synchronous signal generating circuit, a synchronous signal detection circuit, a modulation circuit, and a demodulation circuit.
When data is recorded in the apparatus shown in FIGS. 9 and 10, the ECC circuit 226 adds an error correction code to data sent from the SCSI control circuit 284. The data added with the error correction code is encoded to a recording code by the formatter circuit 227, and the recording code is added with a synchronous signal, which is required in reproduction. Thus, the recording code is sent to the magnetic head driver 281 as a recording signal 228. The magnetic head driver 281 drives the magnetic head 282 according to the input recording signal 228, thereby recording data on the magnetooptical disk 271.
When verification is performed, the data recorded portion of the disk is scanned with a light beam again, as described above, and a recorded signal is read out by the photodetector 277. The readout signal is binarized by the binarizing circuit 278, and the data separator 279 extracts a reproduction signal 229 from the binary data in synchronism with clocks. The extracted reproduction signal 229 is supplied to the formatter circuit 227 in the ODC 280, and various synchronous signals are detected by the synchronous signal detection circuit. The reproduction signal 229 is separated into data and synchronous signals, and only data is input to the ECC circuit 226. The synchronous signals to be used are each constituted by a pattern having a certain redundancy so as to be able to be detected even if they include errors more or less. Even when the synchronous signals are not detected, data can be normally reproduced by, e.g., interpolation.
The ECC circuit 226 performs a syndrome calculation of the input data to detect errors. The ECC circuit 226 counts the number of detected errors. When the number of errors is equal to or smaller than the predetermined determination criterion, the ECC circuit determines that recording is normally performed; otherwise, it determines that a recording error occurred.
On the other hand, in place of performing recording and verification by scanning a light beam on the disk twice, as described above, a so-called direct verifying method for performing verification simultaneously with recording is proposed. As the direct verifying method, a method using a plurality of light beams, as described in Japanese Laid-Open Patent Application No. 58-17546 and a method using reflected light of a recording light beam, as described in Japanese Laid-Open Patent Application No. 62-54857 or 3-73448 are known.
When the above-mentioned method of determining a recording error is applied to the direct verifying method using a plurality of light beams, the following problems are posed. That is, in the direct verifying method, verification is achieved in such a manner that a recording/reproduction light beam and a verifying light beam are arranged to be separated at a predetermined distance in the scanning direction of the light beams. Data recorded by the leading recording/reproduction light beam is read out by the trailing verifying light beam, and the readout data is compared with data delayed by a time corresponding to the predetermined distance. When the timing between the readout data and delayed data is shifted due to, e.g., a variation in rotation speed of the disk, verification cannot be normally performed.
When optical adjustment is performed to optimize the focusing state of the recording/reproduction light beam of the plurality of light beams, the focusing state of the verifying light beam becomes inferior to that of the recording/reproduction light beam due to various aberrations such as a curvature of field of an optical system. The signal-to-noise (S/N) ratio of a signal read out by the verifying light beam in such a focusing state is decreased by about 2 dB as compared to that of a signal reproduced by the recording/reproduction light beam. When the initial performance of the disk and the apparatus is maintained, the decrease in S/N ratio poses no problem. However, in consideration of a change as a function of time, in the worst case, a recording state, which is satisfactory in normal reproduction, may be determined as an error in verification.
For example, when a long distance code (LDC) is used in ECC, a byte error rate before correction, which is necessary for satisfying an error rate of 1.times.10.sup.-12 after error correction is about 2.times.10.sup.-3. This value corresponds to a reproduction limit using the recording/reproduction light beam after passage of time. In contrast to this, the error rate of data read out by the verifying light beam is worsened by 2 dB, as described above, and is about 7.times.10.sup.-3. With this value, if the number of data per sector is 639 bytes, 4 to 5 errors occur per sector even if a disk has no defects. As a result, a sector that does not pose any problem in normal recording may be determined to be defective sector.
On the other hand, when verification is performed using a recording light beam reflected by a disk, data read out by the reflected light represents data which should be recorded in principle, and it is difficult to detect errors due to defects on the disk. When defects are present on the disk in practice, they are overlooked with high possibility. Therefore, when recording error determination is performed by the verifying method using the recording light beam reflected by the disk, the determination result of verification becomes better than an actual recording state contrary to the case wherein a plurality of light beams are used.